Stereoscopic 3d projection systems have been used for many years. One technology known to the art and described for example in US patent no. 2006/0291053A1 dated 28 Dec. 2006 and entitled “Achromatic Polarization Switches”, describes how a polarization modulator can be placed in-front of a single-lens projector such as a 3-chip DLP digital cinema projector.
The projector is arranged so as to generate a single-beam comprising a succession of alternate left and right-eye images at high speeds of typically 144 Hz. The polarization modulator then imparts an optical polarization state to images generated by said projector and said polarization modulator is operated in synchronization with said projector in order to ensure all left-eye images possess a first state of circular polarization and all right-eye images possess a second state of circular polarization, with said first and second states of circular polarization being mutually orthogonal (i.e possessing opposite senses of circular rotation).
Thereafter, said left and right-eye images are focused onto the surface of a polarization-preserving projection-screen such as a silver-screen or otherwise, thereby enabling the viewing of time-multiplexed stereoscopic 3d images via utilization of passive circular-polarized viewing-goggles.
Moreover, it will be known to one skilled in the art that said polarization modulator may comprise of at least one or more liquid crystal elements stacked together in order to achieve the required electro-optical switching characteristics. One technology known to the art and described for example in U.S. Pat. No. 7,760,157 B2 dated 20 Jul. 2010 and entitled “Enhanced ZScreen modulator techniques”, describes how said polarization modulator may comprise of two individual liquid crystal pi-cells stacked together in mutually crossed orientation. Pi-cell liquid crystal elements are known to the art and characterized by their surface alignment-directors on each substrate being mutually parallel and aligned in the same direction. Therefore, in at least one optical state the liquid crystal material in said pi-cell forms a helical structure between said substrates with an overall twist of 180 degrees (i.e pi or π radians). A detailed description of the design and function of pi-cells can be found elsewhere in the literature according to the prior-art.
In this case, each pi-cell can for example be rapidly switched between a first optical state possessing predominantly zero optical retardation when driven with high voltage (eg. 25 volt) in order to switch the liquid crystal material to the homeotropic texture, and a second optical state possessing an optical retardation close to 140 nm (nanometers) when driven with a low voltage (eg. 3 volt) in order to switch the liquid crystal material to the splay texture with predominantly zero degree twist. Moreover, said pi-cells are capable of being rapidly switched between said first and second optical states at speeds of greater than typically 350 μs (microseconds) and are therefore often used when designing such polarization modulator products according to the state of the art.
Furthermore, it will be known to one skilled in the art that when said pi-cell is in an optical state that possesses a retardation value close to 140 nm, then said pi-cell constitutes an optical Quarter-Wave-Plate (QWP) for the central part of the visible wavelength region and will therefore convert linearly polarized visible light directly to circular polarization.
Therefore, by stacking together two individual pi-cells in mutually crossed orientation together with a linear polarization-filter located at the input surface of said stack in order to convert the initially randomly polarized (i.e unpolarized) incident light generated by a projector to linear polarization, the images generated by said projector can be rapidly modulated between left and right circular polarization states by driving said pi-cells mutually out-of-phase such that when said first pi-cell is operated with high voltage (i.e liquid crystal material is switched to the homeotropic texture) then said second pi-cell is simultaneously operated with low voltage (i.e liquid crystal material is switched to the splay texture), and vice versa.
Moreover, it is known to one skilled in the art that the two lenses present in passive circular-polarized viewing-goggles typically each comprise of a linear polarization-filter laminated together with a single uniaxially stretched optical retardation-film. Furthermore, said retardation-film typically possesses an in-plane optical retardation value of substantially 140 nm in order to constitute a Quarter-Wave-Plate (QWP) for the central part of the visible wavelength region. This ensures light that is initially circularly polarized will first be converted to linear polarization by said retardation-film (QWP), before being either transmitted or blocked thereafter by said polarization-filter depending upon the orientation of said linear polarization state.
Moreover, it will be known to one skilled in the art that the linear polarization-filters present in both lenses of common passive circular-polarized viewing-goggles are typically both oriented with their transmission axes aligned horizontally. Furthermore, for the left-eye lens the optical-axis of said retardation-film (QWP) is typically aligned at −45 degrees (minus) in a clockwise direction relative to the horizontal, whereas for the right-eye lens the optical-axis of said retardation-film (QWP) is typically aligned at +45 degrees (plus) relative to the horizontal, respectively.
This ensures light that is initially left circularly polarized (i.e with anti-clockwise sense of rotation) will be transmitted by the right-eye lens whilst simultaneously being blocked by the left-eye lens, whereas light that is initially right circularly polarized (i.e with clockwise sense of rotation) will instead be blocked by the right-eye lens whilst simultaneously being transmitted by the left-eye lens, respectively.
Furthermore, it will be known to one skilled in the art that when the retardation-film (140 nm) present in one of the lenses of said viewing-goggles is mutually crossed with the retardation (140 nm) present in one of said pi-cells being operated with low voltage (i.e when the liquid crystal material is switched to the splay texture), then a high level of optical compensation will occur for all visible wavelengths.
Furthermore, if in addition the linear polarization-filter located at the input surface of said polarization modulator is aligned perpendicularly (i.e with its transmission axis being vertical) relative to the linear polarization-filter present in the lens of said viewing-goggles, then a high level of optical blocking will be achieved for all visible wavelengths, thereby providing for a low level of ghosting or crosstalk when viewing stereoscopic 3d images; this is therefore a preferred arrangement according to the prior-art technology.
Furthermore, when the retardation-film (140 nm) present in one of the lenses of said viewing-goggles is instead mutually parallel with the retardation (140 nm) present in one of said pi-cells being operated with low voltage, then the overall combined retardation will then summate to 140 nm (pi-cell)+140 nm (viewing-goggles)=280 nm, and the system thus constitutes a chromatic Half-Wave-Plate (HWP) for the central part of the visible wavelength region (i.e green wavelengths).
In this case, linearly polarized visible light passing through the system will now be rotated by approximately 90 degrees due to said chromatic Half-Wave-Plate. Additionally, should the linear polarization-filter located at the input surface of said polarization modulator also be aligned perpendicularly (i.e with its transmission axis being vertical) relative to the linear polarization-filter present in one of the lenses of said viewing-goggles, then said lens will transmit the light with high efficiency; this is therefore also a preferred arrangement according to the prior-art.
However, if instead the linear polarization-filter located at the input surface of said polarization modulator is parallel (i.e with its transmission axis being horizontal) relative to said linear polarization-filter present in the lens of said viewing-goggles, then when the retardation present in one of said pi-cells and one of the lenses in said viewing-goggles summate to form a chromatic Half-Wave-Plate, then a high level of ghosting or crosstalk will occur when viewing stereoscopic 3d images since in this case said chromatic Half-Wave-Plate is unable to fully rotate all visible wavelengths by exactly 90 degrees.
It is therefore desirable to avoid utilizing this specific disadvantageous arrangement and instead ensure that when stacking together two pi-cells in mutually crossed orientation in order to design a polarization modulator according to the state of the art, then the linear polarization-filter located at the input surface of said polarization modulator is preferably aligned perpendicularly relative to the linear polarization-filter present in both lenses of said circular polarized viewing-goggles.
Furthermore, since the transmission axes of the linear polarization-filters present in said lenses of common passive circular-polarized viewing-goggles are typically both aligned horizontally, then it will be known to one skilled in the art that one preferred arrangement according to the state of the art is that the transmission axis of the linear polarization-filter located at the input surface of said polarization modulator should be aligned vertically in order to ensure a low level of ghosting or crosstalk is obtained when viewing time-multiplexed stereoscopic 3d images.
The majority of polarization modulator products currently on the market therefore utilize two pi-cell liquid crystal elements stacked together in mutually crossed orientation together with a linear polarization-filter located at the input surface of said polarization modulator and with the transmission axis of said linear polarization-filter being aligned vertically.
However, one problem of the aforementioned single-beam system according to the state of the art is that since images generated by typical 3-chip DLP digital cinema projectors are initially randomly polarized, then the linear polarization-filter located at the input surface of said polarization modulator will absorb approximately 50% of the incoming light generated by said projector. This will therefore significantly reduce the overall optical light efficiency of the system, thereby resulting in the creation of stereoscopic 3d images that are severely lacking in on-screen image brightness.
One technology known to the art for increasing the overall optical light efficiency of a stereoscopic 3d projection system and described for example in U.S. Pat. No. 8,220,934 B2 dated 17 Jul. 2012 and entitled “Polarization conversion systems for stereoscopic projection”, uses a polarization beam-splitting element in order to split the incoming randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating in the same direction as said original incident image-beam and possessing a first state of linear polarization, and one secondary image-beam propagating in a perpendicular direction relative to said incident image-beam and possessing a second state of linear polarization, with said first and second states of linear polarization being mutually orthogonal.
Thereafter, a mirror is used to reflect said secondary image-beam towards the surface of a projection-screen and both primary and secondary image-beams are thereby arranged so as to mutually overlap to a substantial extent on the surface of said projection-screen. Such double-beam systems therefore enable both polarization components comprising the initial incident image-beam to be used in order to generate the overall on-screen image, thereby increasing the resulting image brightness.
Furthermore, polarization modulators are then placed within the optical-paths of both primary and secondary image-beams and designed so as to modulate the polarization states of said image-beams. In one preferred embodiment of the aforementioned double-beam system according to the state of the art, said polarization modulators each comprise of two separate pi-cell liquid crystal elements stacked together in mutually crossed orientation and designed to rapidly modulate the linear polarization states of said primary and secondary image-beams between a left and right circular polarization state in synchronization with the images generated by said projector.
However, in order to obtain a low level of ghosting or crosstalk when utilizing pi-cells of the type described herein, it is stipulated in the aforementioned U.S. Pat. No. 8,220,934 B2 that the linear polarization state of said primary and secondary image-beams at the input surfaces of each polarization modulator must both be aligned perpendicularly (i.e input polarization is required to be vertical) relative to the linear polarization-filters present in both lenses of said passive circular-polarized viewing-goggles.
However, since said primary and secondary image-beams possess mutually orthogonal linear polarization states, it is described in the aforementioned U.S. Pat. No. 8,220,934 B2 that this criterion can only be achieved by using a polarization rotator placed within the optical-path of the secondary image-beam and designed so as to rotate by 90 degrees the linear polarization state of said secondary image-beam so that it is transformed into the same linear polarization state as that of the primary image-beam; i.e the polarization rotator ensures that both primary and secondary image-beams thereafter possess a vertical linear state of polarization which is perpendicular to the transmission axis of the linear polarization-filters present in both lenses of said passive circular-polarized viewing-goggles.
In order for this criterion to be fulfilled, it will be understood by one skilled in the art that the polarization rotator must be placed within the optical-path of the secondary image-beam and be located somewhere between the beam-splitting element and input surface of said polarization modulator, but may be positioned either before or after the reflecting mirror. Moreover, in the case said polarization rotator comprises of several individual elements stacked together, some elements may for example be positioned before said mirror with other elements being positioned after said mirror, respectively.
Whilst the use of a polarization rotator to rotate by 90 degrees the linear polarization state of the secondary image-beam ensures the system possesses a low level of ghosting or crosstalk when viewing stereoscopic 3d images according to the state of the art, the optical efficiency of said polarization rotator is typically less than approximately 90% over the visible wavelength range, thereby resulting in a loss of optical light efficiency and a reduction in the overall on-screen image brightness.
The double-beam system described above in the aforementioned U.S. Pat. No. 8,220,934 B2 according to the state of the art also has the disadvantage in that there is a relatively large optical-path-length difference between said primary and secondary image-beams, thereby typically requiring the use of a telephoto-lens pair and/or the deformation of the reflecting-mirror in order to compensate for said optical-path-length difference. However this will add both complexity and expense to the overall system.
An improved multiple-beam system for displaying high brightness stereoscopic 3d images disclosed in French patent no. FR3000232A1 dated 29 May 2013 and entitled “Dispositif de polarisation optique pour un projecteur d′images stereoscopiques” and incorporated by way of reference herein, uses a beam-splitting element that separates the randomly polarized incident image-beam generated by a single-lens projector into one primary image-beam propagating in the same direction as said original incident image-beam and possessing a first state of linear polarization, and two secondary image-beams propagating in mutually opposite directions which are both perpendicular to said incident image-beam and both possessing a second state of linear polarization, with said first and second states of linear polarization being mutually orthogonal.
Thereafter, reflecting surfaces such as mirrors or otherwise are used to direct both secondary image-beams towards a polarization-preserving projection-screen and arranged such that said primary and secondary image-beams partially overlap in order to mutually recombine to form a complete image on the surface of said projection-screen. Such triple-beam systems therefore enable both polarization components comprising the original incident image-beam to be used to recreate the overall on-screen image, thereby ensuring for a high level of image brightness.
Furthermore, polarization modulators are then placed within the optical-paths of each of said primary and secondary image-beams and operated so as to modulate the polarization states of said image-beams in synchronization with the images generated by said projector.
In one preferred embodiment of the aforementioned triple-beam system, said polarization modulators may each comprise of two individual pi-cell liquid crystal elements stacked together in mutually crossed orientation and operated so as to convert the linear polarization states of said primary and secondary image-beams to circular polarization.
Furthermore, since the linear polarization states of said primary and secondary image-beams are mutually orthogonal, it will be understood by one skilled in the art that the linear polarization state of at least one of said primary and secondary image-beams will be parallel with the transmission axis of the linear polarization-filters present in the lenses of said passive circular-polarized viewing-goggles and in this disadvantageous configuration according to the state of the art there will normally be an undesirably high level of ghosting or crosstalk when viewing time-multiplexed stereoscopic 3d images.
Moreover, in order to mitigate this problem it is known to one skilled in the art that a polarization rotator placed within the optical-paths of said secondary image-beams and located somewhere between the beam-splitter and input surface of said polarization modulators can be used in order to rotate by 90 degrees the linear polarization state of said secondary image-beams so that said linear polarization state is transformed into the same linear polarization state of said primary image-beam. However, since the optical efficiency of said polarization rotator is typically less than approximately 90% over the visible wavelength region, this will generate an unwanted loss of optical light efficiency and reduce the overall on-screen image brightness.
It will also be understood by one skilled in the art that the aforementioned triple-beam system will possess a relatively small optical-path-length difference between said primary and secondary image-beams as compared to the previously described double-beam system according to the state of the art, thereby eliminating the necessity of utilizing a telephoto-lens pair in order to compensate for said optical-path-length difference, hence reducing the overall complexity and cost of the system.